Needleloom, weaving method, and textile articles formed thereby

ABSTRACT

A method of weaving tubular textile articles, comprising: forming first, second, third and fourth superposed layers of warp threads; weaving by weft insertion through sheds formed in said layers, the weaving being performed by first and second weft threads inserted by first and second needles from one side of said warp layers; each weft thread being inserted alternately through a selected pair of said warp layers; and the weft loops at the other side of said layers being knitted together, the first layer with the second layer and the third layer with the fourth layer, to form a pair of selvedges. A tubular textile article formed by the method is provided. A needleloom for weaving such articles is also provided.

The present invention concerns a needleloom able to produce tubulartextile articles; in particular tubular bifurcated grafts for medicaluse. Also provided are a method of weaving and the tubular textilearticles produced thereby.

Bifurcated woven grafts are used for bypass of the aorta and iliacarteries. These grafts have traditionally been woven on a shuttle loomusing two or more shuttles for each weaving head. A shuttle loom reliesupon the shuttle (yarn-package carrier) being passed through the shed(i.e. the opening formed by separating warp threads during the operationof weaving) to insert the weft yarn. The shuttle will carry sufficientweft for many picks. Shuttle loom weaving suffers from several problems,but by far the most important drawback is that of poor yield. An overallyield as low as 10% is not uncommon with shuttle weaving, with evenworse figures for larger sized pieces. This problem is compounded by thefact that shuttle looms are intrinsically slow manufacturing machines.The disadvantages of the shuttle loom are mainly due to the fact that alarge shed is required for the through passage of the boat shuttlethrough the warps. In other words, warp threads need to be separated bya relatively large angle to create sufficient distance between thethreads to allow passage of the shuttle. This leads to a high peaktension in the warp threads, which in turn causes dirt to be transferredto the warp ends from the needle wires. A large shed also leads togreater warp end breakage and yarn filamentation. There has never beenany satisfactory solution to the difficulties.

For almost all textiles, alternatives such as needleloom weaving,knitting or felting have largely replaced shuttle loom weaving. However,since neither knitting nor felting can provide grafts of sufficientdensity and consistent quality, and since technical difficulties have sofar precluded the use of needleloom weaving, shuttle loom weaving is theonly methodology used to date to produce woven bifurcated tubularmedical grafts.

A needleloom is a shuttleless loom in which the weft yarn is drawn froma stationary supply and introduced into the shed by a weft yarninsertion needle with the weft yarn disposed in the form of a doublepick (i.e. the weft yarn is doubled back from the leading end of theweft yarn insertion needle). The weft is retained at the oppositeselvedge by the action of knitting, or by the introduction of a lockingthread from a separate supply. Whilst simple (unbifurcated) tubularmedical grafts can be produced using needleloom technology, technicaldifficulties have prevented this approach being used successfully forbifurcated tubular grafts.

The present invention provides apparatus and methodology able toovercome those technical difficulties.

The present invention provides a method of weaving tubular textilearticles, comprising:

-   -   forming first, second, third and fourth superposed layers of        warp threads;    -   weaving by weft insertion through sheds formed in said layers,        the weaving being performed by first and second weft threads        inserted by first and second needles from one side of said warp        layers;    -   each weft thread being inserted alternately through a selected        pair of said warp layers; and    -   the weft loops at the other side of said layers being knitted        together, the first layer with the second layer and the third        layer with the fourth layer, to form a pair of selvedges.

In a first mode of operation, the first weft thread is insertedalternately through the first and second warp layers, and the secondweft thread is inserted alternately through the third and fourth warplayers, to form two superposed tubes.

In a second mode of operation, the first weft thread is insertedalternately through the first and fourth warp layers, and the secondweft thread is inserted alternately through the second and third warplayers, to form a single tube folded in a C-shape.

The invention further provides a method of weaving a bifurcated tubulartextile article, comprising weaving a pair of tubes by the first of theabove modes of operation, followed or preceded by weaving a single tubeby the second of the above modes of operation using the same warp andweft threads.

The weft loops may be knitted through each other, or knitted togetherwith a binder thread.

Preferably, the tubular article is a surgical or veterinary graft, mostpreferably being bifurcated and forming an aortic or iliac graft.

From another aspect, the present invention resides in a needleloom forweaving tubular textile articles, comprising:

warp yarn disposal means for disposing warp yarns in superposed first,second, third and fourth warp yarn layers;

-   -   shed-forming means for forming a shed in each of said warp        layers;    -   first and second weft insertion needles for inserting first and        second weft threads from one side of said warp layers;    -   upper and lower selvedge knitting means at the other side of the        warp layers for knitting together weft loops formed at the first        and second warp layers and the third and fourth warp layers,        respectively; and    -   control means operable to cause the needleloom to operate        selectively in one of two modes, a first mode passing the first        weft thread alternately through the first and second warp layers        and the second weft thread alternately through the third and        fourth warp layers thereby to form two superposed tubes, and a        second mode passing the first weft thread alternately through        the first and fourth warp layers and the second weft thread        alternately through the second and third warp layers thereby to        form a single tube folded in a C-shape.

In a preferred form, the first and second weft insertion needles arelocated one above the other with a similar spacing to the spacingbetween the warp layers, and the control means is operable to causerelative vertical movement between the weft insertion needles and thewarp layers.

In one preferred form, the first weft insertion needle is alternatelyaligned with the first and second warp layers, the second weft insertionneedle is alternately aligned with the third and fourth warp layers, andwhen operating in said second mode the weft threads are interchangedbetween the first and second weft insertion needles in synchronism withsaid relative movement.

Preferably also, the first weft thread passes through a first weftselector and the second weft thread passes through a second weftselector which is located closer to the warp layers than said first weftselector.

It is to be noted that looms are commonly operated such that the weftyarns form a layer which is substantially horizontal in a directiontransverse to the longitudinal extent of the warp yarns such that thereis an inherent “up” and “down” (as defined by natural gravity) andconsequently two or more superimposed layers of warp yarns automaticallyhave an “upper” and a “lower” in respect of their relative dispositions.However, since operation of the needleloom in accordance with theinvention is independent of gravity, the use of the terms “upper” and“lower” are arbitrary.

The needleloom of the present invention is especially suitable forproduction of medical and veterinary grafts, and in particular forvascular grafts. The needleloom may be used for weaving a bifurcatedtubular graft. However, the needleloom of the present invention is notlimited to weaving bifurcated tubular grafts alone; by remaining in thesecond mode of needleloom operation the needleloom also permits weavingof tapered tubular grafts, in particular where the tapers slopebilaterally symmetrically from both lateral edges of the tubular graft.Further, by remaining in the first mode of needleloom operation, theneedleloom simultaneously weaves two relatively narrow tubular grafts,thus doubling output in comparison to the weaving of a single relativelynarrow tubular graft.

Desirably the method described above uses a Muller System II selvedge(where the weft is interlaced with a binder thread) or a Muller SystemIII selvedge (where the weft yarn and binder thread are interlacedtogether in one go). Muller System II selvedges produce a thinner edgeand are less bulky, whereas the Muller System III selvedge, althoughthicker, is more run proof.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings wherein:

FIGS. 1 a and 1 b are cross-sections respectively of the legs and of thebody of a bifurcated tubular graft, the cross-sections being transverseto the weaving direction which is into the plane of the drawing;

FIG. 2 illustrates the interchange between the two weft yarns used inweaving the body of FIG. 1 b;

FIG. 3 illustrates a needle suitable to interchange the weft yarns at orvery near the stop point for the weft needles;

FIG. 4 illustrates how the weft yarns might catch with one anotherwithout proper arrangement;

FIG. 5 illustrates two plates used to separate the woven article by anamount equal to the needle spacing;

FIGS. 6 a-6 f depict successive stages in the needleloom weaving of thelegs of the vascular graft; and

FIGS. 7 a-7 g depict successive stages in the needleloom weaving of thebody of the vascular graft.

FIG. 8 a shows a conventional design of shed for a needleloom. FIG. 8 bshows a modified design of shed enabling operation of the twinneedleloom of the present invention to manufacture a single body in theform of a four-layered graft.

It should be noted that, in cross-section, the grafts would be held flatby plates but, for clarity of illustration, the grafts are shown-so thateach thickness of cloth can be determined.

Referring first to FIGS. 1 a and 1 b, a bifurcated tubular graft iswoven folded over such that one leg 2 weaves flat on top of the otherleg 3 (FIG. 1 a) and the body 4 is folded along its middle (FIG. 1 b) toform a four-layered graft. Weaving of the legs 2, 3 according to FIG. 1a is straightforward and can also be achieved with standard weavingtechniques but weaving of the body 4 presents many problems.

The solution of the present invention is to weave the body 4 with twoweft yarns 5, 6 (FIG. 2) where one weft yarn 5 alternately weaves thetop layer 7 and the bottom layer 8 of the four-layered graft whilst thesecond weft 6 alternately weaves the two centre layers 10, 9. Thisrequires that the two weft yarns 5, 6 can interchange with one another.In known methodology, three weft yarns would be required, a first weftyarn for the body, a second weft yarns for one leg of the biforcategraft and a third weft yarn for the other leg of the biforcate graft,although these weft yarns would not interchange in the manner envisagedin the present invention.

Two forked weft needles to catch the changed wefts on entry were triedbut the shedding did not permit such a broad front to the weft needle.Two weft yarn insertion needles were therefore tried, each in the formof a needle 11 as shown in FIG. 3 and arranged to interchange the wefts5, 6 at or very near the stop point for the weft yarn insertion needleswhen out of the shed. This requires the top weft yarn insertion needleto accept a weft from underneath rather than from above which is normalpractice. This is described further with reference to FIGS. 7 a and 7 g.The important features of the weft yarn insertion needle 11, which areprovided by modification of commercially available weft yarns insertionneedles, consist of the free end 19 of the needle 11 with a dovetailnotch 17 shaped and dimensioned to carry a weft yarn (not shown in FIG.3) during shed-penetrating movements.

The diameter and length of thee weft yarn insertion needle 11 arestandard, and are dictated to conform with the weaving loom itself. Themodification of the weft yarn insertion needle 11 so that it is suitablefor use in the present invention concerns the radius of the curvature ofthe needle 11 and the depth and spacing of the teeth 18, 18′ forming thenotch 17. Essentially the radius of curvature is increased so that theneedle is less bent relative to a conventional needle. Essentially, theshape of the weft yarn insertion needle is changed to bring the free end19 and notch 17 as close as possible but without touching the weftselectors at the end of each weft insertion cycle.

An appropriate shape for a conventional needle is shown in dottedoutline in FIG. 3 for comparison. Additionally, the spacing betweenteeth 18, 18′ is increased relative to that of a conventional needle tofacilitate the exchange of weft and the depth of notch 17 is increasedto ensure that the wefts remain securely within the notch 17.

There is the real possibility that the wefts 5, 6 for the body 4 willcatch with one another at the entry point to the warps making across-section as depicted in FIG. 4 The solution in this embodiment ofthe invention is to ensure that the weft yarn 6, which is weaving theinner layers of the body 4, is always in the weft selector nearer to thecloth being woven.

Weft yarn 5 which weaves the top and bottom layers 7, 8 requires lessweft yarn when weaving bodies compared with the yarn requirement whenweaving legs and the second weft yarn 6 correspondingly requires moreyarn. A semi-positive weft feed (as opposed to a positive weft feed)accommodates these varying requirements.

When weaving on a twin needle loom of this embodiment of the inventionit is necessary for mechanical reasons for there to be a vertical gap ofat least five millimetres between the selvedge knitting needles 14 and16 (FIGS. 2, 4, and 5) and a similar gap between the weft insertionneedles (not shown in FIGS. 2, 4 and 5). Such a gap would cause there tobe a threadbare section at the entry point of the tube (ie. where theweft insertion needles enter), particularly when weaving graft bodies. Afirst plate 12 (FIG. 5) largely solves this problem by closing the entrygap to a minimum. To ensure that in the worst case where graft bodiesare woven there is no threadbare section, it is necessary to redesignthe drafting of the warps as shown in Appendix 1.

Normally, when weaving on a twin needle loom, the upper two layers areformed by the upper weft and constantly pull upwards and the lower twowith the lower weft pull downwards during shedding to keep the verticalpositions of the cloth fells constant. This is the case when weavinglegs and is important for consistent weaving. However, for weaving ofthe body, the weft yarns regularly interchange their positions and tokeep the cloth fells at constant heights a second plate 13 is inserted(FIG. 5).

With the two plates 12 and 13 in position the shed and heddle wires aremodified to allow a clean passage of the weft yarns. FIG. 8 a shows inschematic form a conventional needle loom shed design whereas FIG. 8 bshows a modification suitable to enable operation of the presentinvention in the formation of a body and/or simultaneous weaving of thelegs. In FIGS. 8 a and 8 b, the shed is the gap described by the upperwarps 14 and lower warps 15. In the modified shed design of FIG. 8 beach weft has both upper and lower warps, the warps having separatebeams 16 and 16′. The woven cloth is formed as 2 separate layers 7, 8.The positioning of the weft insertion needle 11 at the fall of the clothis indicated for clarity. It should be noted that the length of the topwarp yarn 14, 14′ of the shed must be equal to the length of the bottomwarp yarn 15, 15′ of the same shed, but there is no requirement in themodified design of FIG. 8 b for both warp yarns 14, 14′ to be of equallengths.

Details of the needleloom weaving of the graft legs 2 and 3 (FIG. 1 a)will now be discussed with reference to FIGS. 6 a-6 f, and details ofthe needleloom weaving of the graft body 4 (FIG. 1 b) will thereafter bedescribed with reference to FIGS. 7 a-7 g.

The needleloom whose operation is essentially a Muller Needleloommodified in various respects about to be detailed, including thedisposition of the warp yarns in four layers and the provision fortransposing two weft yarns between two weft yarn insertion needles atselected instances in the cycle of needleloom movements. For the sake ofclarity, only those parts of the needleloom essential for explaining theweaving method of the invention are illustrated in FIGS. 6 a-7 g, andthe greater part of the needleloom is omitted from the drawings.

Each of FIGS. 6 a-7 g is a cross-section of the warp yarns transverse tothe direction in which the tubular article is being woven, which isvertically down into the plane of the drawings. For the purposes of thisdescription of this invention, “up” is towards the top of any individualFigure, and “down” is towards the bottom of any individual Figure, withthe relative terms “upper” and “lower” being construed accordingly.Correspondingly, use of the terms “left” and “right” accord with thesame directions in the individual Figures.

At each of the successive stages depicted in FIGS. 6 a-7 g, the warpyarns are divided into four mutually distinct layers which aresuperimposed into a stack of warp yarn layers, each of these four layersbeing vertically subdivided in turn into two sub-layers which togetherform a shed for that layer. (Although each of these sub-layers is a rowof warp yarns viewed in transverse cross-section and should strictly bedepicted as a row of dots or small circles, for simplicity eachsub-layer of warp yarns is depicted as a single continuous horizontalline). At appropriate instants in the cycles of needleloom movementsabout to be detailed, the two sub-layers of each layer of warp yarnshave their respective positions mutually interchanged so as properly tointerleave the respective weft yarn through the warp yarns at thatlayer. (Needleloom components for disposing the warp yarns in fourlayers, and for forming sheds in each of these layers, are not shown inthe drawings). It is to be noted that although the two sub-layers ineach warp yarn layer regularly mutually interchange their respectivepositions, the layers as a whole do not change their relative positionswithin the stack.

Referring to FIG. 6 a in particular, the warp yarns are disposed in astack 20 of four mutually distinct and equidistantly superimposedlayers, namely an upper outside layer 22, an upper inside layer 24, alower inside layer 26, and a lower outside layer 28. Each of these fourlayers is sub-divided by the shed-forming means (not shown) into arespective pair of sub-layers whose vertical positions with respect tothe other sub-layer within each pair of sub-layers are mutuallyinterchanged by the shed-forming means at appropriate moments in thecycle of needleloom movements to allow the interweaving of a first weftyarn 30 or a second weft yarn 32 at appropriate stages in the weavingcycle, as will be detailed below.

To the left of the stack 20 are a pair of movably mounted weft insertionneedles, namely an upper needle 34 and a lower needle 36.

The needles 34 and 36 each engage with the first and second weft yarns30 and 32 respectively to insert the respective weft yarn into the shedformed between the sub-layers of a selected one of the four warp yarnlayers 22, 24, 26 and 28 (as detailed below). The needles 34 and 36 aremutually mechanically linked so as to move conjointly in a lateraldirection. When the needleloom is operating in its first mode ofoperation to weave the graft legs 2 and 3 (as detailed in FIGS. 6 a-6f), the first weft yarn 30 remains continuously engaged with the upperweft insertion needle 34 and the second weft yarn 32 remainscontinuously engaged with the lower weft insertion needle 36. However,when the needleloom is operating in its second mode of operation toweave the graft body 4 (as detailed in FIGS. 7 a-7 g), the first weftyarn 30 is, at various parts of the weaving cycle, either engaged withthe upper weft insertion needle 34 (FIGS. 7 a, 7 b, 7 c and 7 g) orengaged with the lower weft insertion needle 36 (FIGS. 7 d, 7 e and 7 f)while the second weft yarn 32 is contemporaneously carried by the one ofthe weft insertion needles 34 and 36 not currently carrying the firstweft yarn 30. (Means for interchanging the first and second weft yarns30 and 32 between the upper and lower weft insertion needles 34 and 36are not shown in the drawings). To the right of the stack 20 are thepair of selvedge knitting needles previously described with reference toFIGS. 2, 4 and 5, namely the upper selvedge knitting needle 14 and thelower selvedge knitting needle 16. During both modes of needleloomoperation, the upper selvedge knitting needle 14 is operated when one orother of the weft yarns 30 and 32 is passed by the upper weft insertionneedle 34 through the respective shed in one or other of the two upperwarp layers 22 and 24 to knit together the adjacent selvedges at theright edge of the two upper weft layers 22 and 24. Also during bothmodes of needleloom operation, the lower selvedge knitting needle 16 isoperated when one or other of the weft yarns 30 and 32 is passed by thelower weft insertion needle 36 through the respective shed in one orother of the two lower warp layers 26 and 28 to join together theadjacent selvedges at the right edge of the two lower warp layers 26 and28.

At all times, the selvedge knitting needles 14 and 16 remain at the sameheight with respect to the warp layer stack 20.

While FIG. 6 a contains reference numerals for all components andmaterials, these reference numerals will be left out of FIGS. 6 b-7 gfor increased clarity, except where one or more reference numerals areconsidered to be necessary or convenient for understanding of particularFigure.

Reverting to FIG. 6 a, this shows the weft yarn insertion needles 34 and36 laterally retracted leftwards away from the warp yarn layer stack 20,with the upper needle 34 trailing the first weft yarn 30 from the shedbetween the two sub-layers of the upper inside layer 24, and with thelower needle 36 trailing the second weft yarn 32 from the shed betweenthe two sub-layers of the lower outside layer 28. (See the subsequentdescription of FIG. 6 f for an explanation of how the arrangement ofFIG. 6 a is arrived at). FIG. 6 a also shows the selvedge knittingneedles 14 and 16 laterally retracted rightwards away from the warp yarnlayer stack 20, with the upper selvedge knitting needle 14 havingimmediately previously knitted a selvedge uniting the adjacent rightedges of the two upper layers 22 and 24, and with the lower selvedgeknitting needle 16 having immediately previously knitted a selvedgeuniting the adjacent right edges of the two lower layers 26 and 28.Following the weaving of layers 24 and 28 the yarn layer stack realignsto weave layers 22 and 26.

Referring now to FIG. 6 b, this illustrates the stage in first-modeneedleloom operation immediately following the previously completedstage described above with reference to FIG. 6 a. As shown in FIG. 6 b,both weft yarn insertion needles 34 and 36 have been moved fullyrightwards to cause the upper needle 34 to penetrate the shed formedbetween the two sub-layers of the upper outside warp yarn 22, and tocause the lower needle 36 to penetrate the shed formed between the twosub-layers of the lower inside warp yarn layer 26. The upper needle 34thereby carries the first weft yarn 30 rightwards through the shed ofthe upper outside layer 22 to the right side of layer 22 where the weftyarn 30 is knitted by the upper selvedge knitting needle 14 with theright edge of the adjacent upper inside layer 24 to unite these twoedges in a common selvedge. Also, the lower needle 36 carries the secondweft yarn 32 rightwards through the shed of the lower inside layer 26 tothe right side of the layer 26 where the weft yarn 32 is joined by thelower selvedge knitting needle 16 with the right edge of the adjacentlower outside layer 28 to unite these two edges in a common selvedge.

Following the weaving and selvedge knitting stage of FIG. 6 b, the weftinsertion needles 34 and 36 are fully withdrawn leftwards out of thelayers 22 and 26 as shown in FIG. 6 c, leaving the first weft yarn 30woven into the upper outside layer 22 and leaving the second weft yarn32 woven into the lower inside layer 26. At the same time, the selvedgeknitting needles 14 and 16 are fully withdrawn rightwards to be clear ofthe newly knitted selvedges.

Turning now to FIG. 6 d, this shows the stack 20 moved bodily upwards.This stack movement brings the upper inside layer 24 level with theupper weft insertion needle 34, and brings the lower outside layer 28level with the lower weft insertion needle 36, so creating thealignments necessary for the next stage in the first mode of needleloomoperation. Requisite movement of the stack can be accomplished by anysuitable procedure.

FIG. 6 e shows the next stage in the first mode of needleloom operation,wherein both weft yarn insertion needles 34 and 36 have been moved fullyrightwards to cause the upper needle 34 to penetrate the shed formedbetween the two sub-layers of the upper inside layer 24, and to causethe lower needle 36 to penetrate the shed formed between the twosub-layers of the lower outside warp yarn layer 28. The upper needle 34thereby carries the first weft yarn 30 rightwards through the shed ofthe upper inside layer 24 to the right side of the layer 24 where theweft yarn 30 is knitted by the upper selvedge knitting needle 14 withthe right edge of the adjacent upper outside layer 22 to unite these twoedges in a common selvedge. Also, the lower needle 36 thereby carriesthe second weft yarn 32 rightwards through the shed of the lower outsidelayer 28 to the right side of the layer 28 where the weft yarn 32 isknitted by the lower selvedge knitting needle 16 with the right edge ofthe adjacent lower inside layer 26 to unite these two edges in a commonselvedge.

Following the weaving end selvedge knitting stage of FIG. 6 e, the weftinsertion needles 34 and 36 are fully withdrawn leftwards out of thelayers 24 and 28 as shown in FIG. 6 f, leaving the first weft yarn 30woven into the upper inside layer 24 and leaving the second weft yarn 32woven into the lower outside layer 28. At the same time, the selvedgeknitting needles 14 and 16 are fully withdrawn rightwards to be clear ofthe newly knitted selvedges.

Following the stage illustrated in FIG. 6 f, the stack 20 is movedbodily downwards. This exactly reverses the upward movement of the stack20 described with reference to FIG. 6 d, and produces the arrangementshown in FIG. 6 a, so completing a full cycle of needleloom movements inthe first mode of needleloom operation.

It is to be noted that beating-up (i.e. forcing the picks of newly wovenweft yarn into the fells) will take place at suitable points in theabove-described sequence of stages (e.g. at the stage shown in FIG. 6 cand/or at the stage shown in FIG. 6 f). Any suitable means forbeating-up may be employed, but such means are omitted from thedrawings.

The cycle of operations described above with reference to FIGS. 6 a-6 fis repeated an appropriate number of times, with appropriate feeding ofthe weft yarns 30 and 32, and winding on from the needle insertionregions of the twin tubes (2 and 3, FIG. 1 a) woven by this first modeof needleloom operation. When a predetermined length of the twin tubeshas been woven, the needleloom is switched to a second mode ofneedleloom operation which will now be described with reference to FIGS.7 a-7 g.

The second mode of needleloom operation results in the weaving of asingle tube which serves as the body 4 (FIG. 1 b) of the graft. Thetransitions from twin tube to single tube, and the alternate transitionsfrom single tube to twin tube, each form a respective crotch in thewoven textile article produced by operation of the needleloom, eachcrotch being the Y-junction in the resultant grafts when cut to lengthfrom the normally continuous alternating single/twin tubing woven by theneedleloom.

FIG. 7 a shows the weft yarn inserting needles 34 and 36 laterallyretracted leftwards away from the warp yarn layer stack 20, with upperneedle 34 trailing the first weft yarn 30 from the shed between the twosub-layers of the lower outside layer 28, and with the lower needle 36trailing the second weft yarn 32 from the shed between the twosub-layers of the upper inside layer 24. (See the subsequent descriptionof FIG. 7 g for an explanation of how the arrangement of FIG. 7 a isarrived at). FIG. 7 a also shows the selvedge knitting needles 14 and 16laterally retracted rightwards away from the warp yarn layer stack 20,with the upper selvedge knitting needle 14 having immediately previouslyknitted a selvedge uniting the adjacent right edges of the two upperlayers 22 and 24, and with the lower selvedge knitting needle 16 havingimmediately previously knitted a selvedge uniting the adjacent rightedges of the two lower layers 26 and 28.

The arrangement of FIG. 7 a corresponds to the arrangement of FIG. 6 aexcept that whereas in the first mode of needleloom operation (FIGS. 6a-6 f), the first weft yarn 30 was woven alternately into the two upperlayers 22 and 24 while the second weft yarn 32 was woven alternatelyinto the two lower layers 26 and 28, in the second mode of needleloomoperation (FIGS. 7 a-7 g), the first weft yarn 30 is woven alternatelyinto the two outside layers 22 and 28 while the second weft yarn 32 iswoven alternately into the two inside layers 24 and 26. (In the secondmode of needleloom operation, respective selvedges continue to mutuallyunite the two upper layers 22 and 24 and to mutually unite the two lowerlayers 26 and 28, in the same manner as in the first mode of needleloomoperation).

Referring now to FIGS. 7 b and 7 c, these stages of the second mode ofneedleloom operation (which follow in succession from the stages shownin FIG. 7 a) correspond to the equivalent stages of the first mode ofneedleloom operation as shown in FIGS. 6 b and 6 c, save for thedifferent starting configuration shown in FIG. 7 a (compare with FIG. 6a).

The next stage of the second mode of needleloom operation as shown inFIG. 7 d demonstrates one of the most significant differences in thesecond mode with respect to the first mode of needleloom operation,namely the transposition of the weft yarns 30 and 32 between the weftinsertion needles 34 and 36 in readiness for the next stage ofneedleloom operation. Whereas the stages shown in FIGS. 7 a, 7 b and 7 chad the first weft yarn 30 carried by the upper weft yarn insertionneedle 34 and the second weft yarn 32 carried by the lower weft yarninsertion needle 36 (i.e. as done throughout the first mode ofneedleloom operation and illustrated in FIGS. 6 a-6 f), the subsequentstages shown in FIGS. 7 d, 7 e and 7 f require the first weft yarn 30 tobe carried by the lower weft yarn insertion needle 36 and the secondweft yarn 32 to be carried by the upper weft yarn insertion needle 34.Weft yarn changeover takes place at the stage shown in 7 d, with theinterchange being conducted by weft selectors (not shown), the weftselector for the second weft yarn 32 being located laterally closer tothe stack 20 than the weft selector for the first weft yarn 30 so as toavoid the unwanted weft yarn entanglement previously mentionedwith-reference to FIG. 4. The weft selection may each consist of aheddle wire arrangement for each weft yarn, with the weft yarn passingthrough an eye in the weft selector. The weft selectors areindependently moveable in a direction traverse to that of weftinsertion. Hence the weft selector carrying the yarn to be inserted intothe upper weft yarn insertion needle 34 moves upwardly (as viewed inFIG. 7) at the moment the upper weft yarn insertion needle 34 is fullyretracted and prior to its next insertion in the cycle. The upwardmovement of the weft selector lifts the yarn out of the lower weft yarninsertion needle 36, and over the upper weft yarn insertion needle 34such that the yarn drops into the notch 17 of needle 34 as that needlecommences its next insertion cycle. Simultaneously the weft selectorcarrying the yarn to be inserted into the lower weft yarn insertionneedle 36 moves that yarn downwardly to facilitate its accurateplacement into notch 17 of the lower weft yarn insertion needle 36. Inthe second mode of needleloom operation, this weft selector is initiallylocated at a position such as to just lift the weft out of the upperweft yarn insertion needle at the end of the insertion cycle. At thesame time as the weft yarn positions are interchanged, the stack 20 isbodily moved upwards.

Following the weft yarn interchange shown in FIG. 7 d, the next stage ofthe second mode of needleloom operation is shown in FIG. 7 e whichcorresponds to the first-mode stage shown in FIG. 6 e except that inFIG. 7 e, it is the second weft yarn 32 that is woven into the upperinside layer 24 and the first weft yarn 30 that is woven into the loweroutside layer 28. (Selvedge knitting continues as before). At theconclusion of FIG. 7 e stage, all the various needles are laterallyretracted as shown in FIG. 7 f (which corresponds to FIG. 6 f).

The final stage of the second mode of needleloom operation isillustrated in FIG. 7 g, wherein the weft yarns 30 and 32 are againtransposed between the weft yarn insertion needles 34 and 36, such thatthe first weft yarn 30 is returned to the upper needle 34 and the secondweft yarn 32 is returned to the lower needle 36. As the same time, thestack 20 is bodily lowered to reverse the upward movement of FIG. 7 d.These movements described with reference to FIG. 7 g return theneedleloom configuration to the starting configuration of FIG. 7 a, andthereby complete the cycle of stages constituting the second mode ofneedleloom operation, i.e. the weaving of a single tube in afolded-double configuration (as previously detailed in FIG. 1 b).

The cycle of operations described above with reference to FIGS. 7 a-7 gis repeated an appropriate number of times, with appropriate feeding ofthe weft yarns 30 and 32, and winding on from the needle insertionregions of the folded single tube (4, FIG. 1 b) woven by this secondmode of needleloom operation. When a predetermined length of the foldedsingle tube has been woven, the needleloom is switched back to its firstmode needleloom operation (as previously described with reference toFIGS. 6 a-6 f).

The drive/control arrangement which produces the alteration of theneedles is standard equipment with commercially available twine needlelooms. The changeover from the production of one tube to two legs andvice versa is easily controlled by programming the control unit of acommercially available twin needleloom.

Modifications and variations of the above-described needleloom andweaving method can be adopted without departing from the scope of theinvention. For example, if the respective positions of the two weft yarninsertion needles 34 and 36 could be mutually interchanged duringneedleloom operation, then the second mode of needleloom operation(FIGS. 7 a-7 g) could be carried out by interchanging the needlepositions at stages 7 d and 7 g without interchanging the weft yarns 30and 32 between the needles 34 and 36.

EXAMPLE 1 Risk Assessment

Currently bifurcate grafts are produced on the Muller Shuttle Loom.These looms are relatively slow, can be unreliable and the graftsproduced on them-can be prone to soiling. It is now intended to startproducing bifurcate grafts on the Muller Needle Loom. This loom canoffer a number of advantages:

-   (1) It takes less time to produce a bifurcate graft.-   (2) It is more reliable, and if there is a problem during    manufacture, the run can be aborted and a new graft manufactured    immediately. This is unlike the shuttle loom, which must complete    the faulty graft before starting a new graft.-   (3) It produces graft with less soiling.

In addition to being produced on a different loom, the grafts from theneedle loom will be produced with a Muller System II selvedge ratherthan the Muller System III selvedge that is used for other woven grafts.The Muller System II selvedge is thinner and less bulky than the MullerSystem III edge.

-   Muller System II Selvedge—Interlacing of the weft with a binder    thread. This type of selvedge has a thinner edge and will be less    bulky.-   Muller System III Selvedge—Interlacing of the weft and binder thread    in one go. This type of selvedge is thicker and run proof.

Testing was conducted to see whether:

-   (1) The grafts produced on the needle loom were as blood tight as    those produced on the shuttle loom.-   (2) The Muller System II selvedge causes blood leakage from the    graft.-   (3) The grafts produced on the needle loom have different physical    characteristics than those manufactured on the shuttle loom.-   (4) The Muller System II selvedge is weaker than the Muller System    III selvedge.    Evaluation-   (1) Bench blood testing was carried out on grafts which have been    produced on the Muller Needle Loom and then gel sealed. Particular    attention was paid to the selvedge area, to ensure that the Muller    System II edge is not having a negative effect. The testing    conducted approximates to ISO7198, paragraph 8.2.3 except that    anti-coagulated animal blood is used as the fluid. Briefly, the    graph is attached to a reservoir of blood held at 120 mmHg by a    regulated air supply. The blood is forced into the graft and any    leakage is noted. Since the volume left is small, observation of    leakage (rather than measurement of volume) is relied upon. The    results are presented in Example 2.-   (2) Physical testing was carried out on grafts produced from the    needle loom. The testing was conducted in accordance with ISO7198 as    detailed in Example 3. These results were compared with previous    results for grafts produced on the shuttle loom. Again particular    attention was paid to the selvedge area of the grafts with regard to    the burst strength and water permeability. The results are presented    in Example 3.-   (3) The tensile strength of the selvedge was determined. This was    carried out by cutting the graft into 2 cm sections; the graft was    then cut longitudinally so that the selvedge was positioned in the    middle of the fabric. The tensile strength was then tested as per    ISO7198, paragraph 8.3.2. The results of the needle loom versus    shuttle loom are presented in Example 3.    Results-   (1) The results of blood testing show that the modifications have    not affected the blood handling properties of the graft.-   (2) The report on physical testing is attached in Example 3. The    results show that the grafts produced on the needle loom are    thinner, stronger in the longitudinal direction and have a lower    porosity than the shuttle loom grafts. The shuttle loom grafts have    a higher burst strength.-   (3) Table 4 in Example 3 compares the tensile strength of the    selvedges. The results show that the Muller System II selvedge is    slightly weaker than the Muller System III.    Conclusion

The blood testing results show that the needle loom grafts performed aswell as the shuttle loom grafts.

Physical testing showed that the needleloom-woven grafts had a lowerburst strength than the grafts woven on the shuttle loom. This lowerburst strength however, was still far in excess of the limits set forbifurcate grafts. The tensile strength of the Muller System II selvedgewas slightly lower than that of the Muller System III selvedge. Thisdifference, although significant, is not high enough to affect theclinical performance of the graft. The needleloom-woven grafts arethinner, stronger in the longitudinal direction and have a lower waterporosity than grafts woven on the shuttle loom.

The additional risks proved by this modification have been identified,addressed by testing and shown to be far outweighed by the benefits ofthe modifications.

EXAMPLE 2 Bench Blood Testing Results

Method

Seven 18 by 9 mm internal diameter needle loom woven bifurcate graftsfrom the same batch were blood tested according to ISO7198, paragraph8.2.3 except that anticoagulated animal blood was used as the testfluid. These grafts were all produced on the Muller needle loom with aMuller System II selvedge. The catalogue number for these grafts was731809 and the batch number 29784. The results of these grafts were thencompared with equivalent grafts produced on the Muller shuttle loom andblood tested in August 1997. The grafts tested were:

-   Cat No.: 732211, batches 25630 and 25682-   Cat No.: 732010, batch 24517B-   Cat No.: 731407, batches 25034/A and 24505/1A    Results    Needle Loom-   Initial Pressurisation—None of the grafts leaked-   First Pull—Two of the grafts did not leak. Of the other five, three    had small spot leak at the crotch of the bifurcate and the remaining    two had a leak on the leg just below the crotch.-   Second Pull—One of the grafts did not leak, the remaining six grafts    had small leaks at the crotch area of the bifurcate.-   Overall Performance—All the grafts performed very well. The leaks,    which did occur, were very small and sealed very quickly. The total    amount of blood lost from each graft was too small to be measured    accurately.    Shuttle Loom-   Initial Pressurisation—Four of the grafts did not leak and the other    had a few small spot leaks on the legs and body of the graft.-   First Pull—Two of the grafts did not leak and the other three had    small crotch leaks.-   Second Pull—Two of the grafts had crotch leaks only while the other    three had between one and three small spot leaks which were mainly    on the legs of the graft.-   Overall Performance—The grafts performed very well with only small    spot leaks occurring on the legs of the graft, which sealed very    quickly. The total amount of blood lost from the grafts was    negligible.    Conclusion

The grafts manufactured on the Muller needle loom performed as well asthose which were manufactured on the Muller shuttle loom. The MullerSystem II selvedge also performed very well and did not cause any bloodloss from the graft.

EXAMPLE 3 Physical Characteristics of Bifurcate Grafts Produced on theMuller Needle and Shuttle Looms

Introduction

The physical properties of bifurcate grafts manufactured on the Mullerneedle loom (Muller System II selvedge) were compared with those ofbifurcate grafts produced on the Muller shuttle loom (Muller System IIIselvedge).

Method

Bifurcate grafts were tested according to the following specificationsof ISO 7198:

-   8.2.2—Determination of water porosity on Buxton & Cooley type rig.-   8.3.3.2—Measurement of product burst strength—body, seam/black line,    crotch.-   8.5—Measuring relaxed internal diameter.-   8.2.3*—Whole graft porosity test.

* 8% glycerol in propanol was substituted for the test fluid.

-   8.8—Suture retention.-   8.3.2—Longitudinal tensile strength.-   8.7.4.2—Wall thickness

The following grafts were tested:

Nine grafts from Batch 29878. These were 18 mm*9 mm bifurcate graftsproduced on the Muller Needle loom. The whole graft porosity of nine 18mm & 9 mm grafts produced on the Muller needle loom (Batch 29784) werealso tested. Physical testing of bifurcate grafts produced on theshuttle loom had already been carried out and the results used as acomparison with the needle loom grafts.

Results

TABLE 1 Burst Strength Results Burst Strength (Newtons) Area of grafttested Needle loom Shuttle Loom Body - normal fabric 403 Body - blackline 322 Leg - normal fabric 388 Leg - black line 321 Overall Mean 359434

The only burst strength data available for the shuttle loom was for theoverall mean.

TABLE 2 Water Permeability Results Water Permeability (ml/cm²/minute)Area of graft tested Needle loom Shuttle Loom Body - normal fabric 223Body - black line 224 Leg - normal fabric 248 Leg - black line 230Overall Mean 231.3 343.3

The only water permeability values for grafts produced on the Mullershuttle looms was the overall mean.

TABLE 3 Other Physical Parameters Parameter Needle Loom Shuttle LoomUnits Suture retention 26.81 25.86 Newtons Longitudinal 21.75 13.56Newtons/mm tensile strength Wall thickness 0.41  0.514 mm (nominal) Wallthickness 0.219  0.312 mm (flat stock) Whole graft 0.06  0.0076*ml/cm²/minute porosity *whole graft porosity of bifurcates tested 11/96

TABLE 4 Tensile Strength of Selvedges of Grafts Produced on Needle andShuttle Looms Loom type Tensile strength (Newtons) Needle loom 181Shuttle loom 208Conclusion

Statistical analysis (Student's t-test) of the results show that withthe exception of the suture retention, the physical parameters of needleand shuttle loom grafts are different. The needle loom grafts havesignificantly lower water permeability, higher longitudinal tensilestrength and a decreased wall thickness. These characteristics wouldenhance the performance of the graft.

The needle loom grafts however, have a weaker burst strength and tensilestrength at the selvedge. The burst strength although weaker was stillwell within the set performance limits.

The difference in the tensile strength of the System II and System IIIselvedges, although significant, was very small. The selvedge strengthis an important factor in the burst strength, longitudinal tensilestrength and blood handling of the graft. As none of these parametersare being affected negatively, the slightly lower selvedge strengthshould not affect the clinical performance of the graft.

APPENDIX 1

1. A method of weaving a bifurcated tubular textile article, comprising:(a) forming first, second, third and fourth superposed layers of warpthreads; (b) weaving by weft insertion through sheds formed in saidlayers, the weaving being performed by first and second weft threadsinserted by first and second needles from one side of said warp layers,each weft thread being inserted alternately through a selected pair ofsaid warp layers, and the weft loops at the other side of said layersbeing knitted together, the first layer with the second layer and thethird layer with the fourth layer, to form a pair of selvedges; (c) thefirst weft thread being inserted alternately through the first andsecond warp layers, and the second weft thread being insertedalternately through the third and fourth warp layers, to form twosuperposed tubes; and (d) either after or before step (c), using thesame warp and weft threads, the first weft thread being insertedalternately through the first and fourth warp layers, and the secondweft thread being inserted alternately through the second and third warplayers, to form a single tube folded in a C-shape; thereby producing abifurcated article.
 2. A method according to claim 1, in which the weftloops are knitted through each other.
 3. A method according to claim 1,in which the weft loops are knitted together with a binder thread.
 4. Amethod according to claim 1, in which the tubular article is a surgicalor veterinary graft.
 5. A method according to claim 4, in which thetubular article is an aortic or iliac graft.
 6. A tubular textilearticle produced by the method of claim
 1. 7. A graft produced by themethod of claim
 1. 8. A needleloom for weaving tubular textile articles,comprising: warp yarn disposal means for disposing warp yarns insuperposed first, second, third and fourth warp yarn layers;shed-forming means for forming a shed in each of said warp layers; firstand second weft insertion needles for inserting first and second weftthreads from one side of said warp layers; upper and lower selvedgeknitting means at the other side of the warp layers for knittingtogether weft loops formed at the first and second warp layers and thethird and fourth warp layers, respectively; and control means operableto cause the needleloom to operate selectively in two modes, one of saidmodes passing the first weft thread alternately through the first andsecond warp layers and the second weft thread alternately through thethird and fourth warp layers thereby to form two superposed tubes, andthe other of said modes passing the first weft thread alternatelythrough the first and fourth warp layers and the second weft threadalternately through the second and third warp layers thereby to form asingle tube folded in a C-shape, said two modes being operatedsequentially thereby to form a bifurcated article.
 9. A needleloomaccording to claim 8, in which the first and second weft insertionneedles are located one above the other with a similar spacing to thespacing between the warp layers, and the control means is operable tocause relative vertical movement between the weft insertion needles andthe warp layers.
 10. A needleloom according to claim 9, in which thefirst weft insertion needle is alternately aligned with the first andsecond warp layers, the second weft insertion needle is alternatelyaligned with the third and fourth warp layers, and when operating insaid second mode the weft threads are interchanged between the first andsecond weft insertion needles in synchronism with said relativemovement.
 11. A needleloom according to claim 10, in which the firstweft thread passes through a first weft selector and the second weftthread passes through a second weft selector which is located closer tothe warp layers than said first weft selector.